Acyl carrier proteins (ACPs) are small (˜8 to 10 kDa) acidic proteins that contain a 4′-phosphopantetheine prosthetic group. This prosthetic group is attached to a conserved serine residue in apo-ACP by holo-ACP synthase. (That is, apo-ACP is converted to holo-ACP by addition of the prosthetic group, a reaction catalyzed by holo-ACP synthase.) In -holo-ACP, the prosthetic group provides a free thiol group that is required for ACP to function in a variety of biosynthetic pathways including de novo biosynthesis of fatty acids,1 depsipeptides,2 peptides,3 polyketides,4 the posttranslational acylation of proteins,5 bacterial quorum sensing,32 the synthesis of intercellular signalling molecules,33 the establishment of biofilms,34 and conversion to virulence.35, 36 
Acyl-ACPs are also substrates for the soluble desaturases found in the plastid organelles of plants and photoauxotrophic Euglena.6 The stearoyl-ACP Δ9 desaturase (Δ9D) from Ricinus communis is the best characterized member of this enzyme family.7 The Δ9D desaturase catalyzes the NADPH- and O2-dependent insertion of a cis-double bond at the C9 position of 18:0-ACP to form 18:1-ACP. Non-heme diiron centers found in each subunit of the homodimeric Δ9D (8-10) are utilized for the O2 activation steps of catalysis.
Recent studies have revealed the importance of protein-protein interactions between acyl-ACP and Δ9D in the determination of catalytic selectivity,11 the perturbation of the ligation environment of the diiron center,12 and the accumulation of a quasi-stable peroxodiiron(III) species. 13, 14 
There is, however, an acute need to elucidate these interactions in greater detail. Further biophysical characterization of the protein-protein interactions involved in these catalytic phenomena would be facilitated by the availability of selective probes for complex formation. Because neither acyl-ACP, nor Δ9D, nor other protein partners that interact with ACP in the above-mentioned biological processes exhibits chromophoric features suitable for these studies, another tack must be taken. The present invention addresses this long-felt need by providing a method to synthesize suitably-derivatized forms of acyl-ACP that allows non-radioactive monitoring of reactions involving ACP. Thus, the present invention is directed to ACPs with a site-specific chromophoric or fluorophoric modification. The labeled ACP can be used a probe and includes the appropriate fatty acyl derivative required for subsequent catalytic reactions.